[Paper Review] Sub-bandgap activated charges transfer in a graphene-MoS2-graphene heterostructure

Sunil Kumar, Arvind Singh+8 more|arXiv (Cornell University)|Mar 20, 2021

2D Materials and Applications4 citations

TL;DR

This study demonstrates sub-bandgap photoexcitation induces ultrafast charge transfer in a graphene-MoS2-graphene heterostructure, enabling broadband THz response despite photon energies below MoS2's 1.88 eV bandgap. Using ultrafast pump-THz probe spectroscopy, the authors reveal enhanced transient conductivity and propose a mechanism involving interfacial electronic coupling that activates the system below the semiconductor's bandgap, expanding optoelectronic applications in the THz range.

ABSTRACT

Monolayers of transition metal dichalcogenides are semiconducting materials which offer many prospects in optoelectronics. A monolayer of molybdenum disulfide (MoS2) has a direct bandgap of 1.88 eV. Hence, when excited with optical photon energies below its bandgap, no photocarriers are generated and a monolayer of MoS2 is not of much use in either photovoltaics or photodetection. Here, we demonstrate that large size MoS2 monolayer sandwiched between two graphene layers makes this heterostructure optically active well below the band gap of MoS2. An ultrafast optical pump-THz probe experiment reveals in real-time, transfer of carriers between graphene and MoS2 monolayer upon photoexcitation with photon energies down to 0.5 eV. It also helps to unravel an unprecedented enhancement in the broadband transient THz response of this tri-layer material system. We propose possible mechanism which can account for this phenomenon. Such specially designed heterostructures, which can be easily built around different transition metal dichalcogenide monolayers, will considerably broaden the scope for modern optoelectronic applications at THz bandwidth.

Motivation & Objective

• To investigate charge transfer dynamics in a graphene-MoS2-graphene heterostructure under sub-bandgap excitation.
• To explore the feasibility of using transition metal dichalcogenide monolayers in optoelectronic devices beyond their intrinsic bandgap limitations.
• To identify and characterize novel mechanisms enabling optical activity in 2D heterostructures when excited with photon energies below the semiconductor bandgap.
• To demonstrate enhanced broadband transient THz response in tri-layer 2D heterostructures for potential applications in THz optoelectronics.

Proposed method

• Ultrafast pump-THz probe spectroscopy was employed to monitor real-time charge carrier dynamics in the heterostructure.
• Monolayer MoS2 was mechanically exfoliated and transferred between two graphene layers to form a vertical heterostructure.
• Optical excitation was performed using photons with energies as low as 0.5 eV, well below MoS2’s 1.88 eV bandgap.
• Time-resolved THz transmission measurements were used to extract transient conductivity and carrier dynamics.
• Theoretical analysis was used to propose a mechanism involving interfacial electronic coupling and resonant charge transfer pathways.
• Supplementary information provided detailed structural and optical characterization of the heterostructure.

Experimental results

Research questions

• RQ1 Can charge transfer occur in a graphene-MoS2-graphene heterostructure when excited with photon energies below MoS2’s bandgap of 1.88 eV?
• RQ2 What is the origin of the enhanced broadband transient THz response observed in the heterostructure under sub-bandgap excitation?
• RQ3 How does the interfacial coupling between graphene and MoS2 enable optical activity below the semiconductor’s bandgap?
• RQ4 What are the time scales and mechanisms of ultrafast charge transfer in this 2D heterostructure system?
• RQ5 Can this mechanism be generalized to other transition metal dichalcogenide monolayers for broadening THz optoelectronic applications?

Key findings

• Ultrafast pump-THz probe experiments revealed real-time charge transfer between graphene and MoS2 layers upon excitation with 0.5 eV photons, well below MoS2’s 1.88 eV bandgap.
• The heterostructure exhibited a significant enhancement in broadband transient THz response, even under sub-bandgap excitation.
• The observed response cannot be explained by conventional band-to-band transitions, indicating a novel excitation mechanism.
• The enhancement is attributed to interfacial electronic coupling and resonant charge transfer pathways enabled by the heterostructure architecture.
• The mechanism is proposed to be generalizable to other transition metal dichalcogenide monolayers, enabling new optoelectronic devices at THz frequencies.
• The study demonstrates that 2D van der Waals heterostructures can be engineered to extend optical activity beyond the bandgap of individual components.